The present invention concerns in general terms digital signal coding, and for this purpose proposes a device and a method for coding a digital signal by breaking down the signal into frequency sub-bands and coding the coefficients resulting from the breakdown into sub-bands. It also concerns a decoding method and device corresponding to the coding method and device.
The purpose of coding is to compress the signal, which enables the digital signal to be transmitted, or respectively stored, thus reducing the transmission time or transmission rate, or respectively reducing the memory space used.
The invention is situated in the field of compression of digital signals with loss.
Breaking down a signal into frequency sub-bands before compressing it is known. The breakdown consists of creating, from the signal, a set of sub-bands which each contain a limited range of frequencies. The sub-bands can be of different resolutions, the resolution of a sub-band being the number of samples per unit length used for representing the sub-band. In the case of a digital image signal, a frequency sub-band of this signal can itself be considered to be an image, that is to say a bidimensional table of numerical values.
It should be noted that breaking down a signal into frequency sub-bands does not create any compression in itself, but makes it possible to decorrelate the signal so as to eliminate redundancy therefrom prior to the compression proper. The sub-bands are thus coded more effectively than the original signal.
A known method of coding a digital signal, in this case a digital image, uses the similarity between blocks of different sub-bands of a multiresolution breakdown of the image. This method is based on an inter-scale prediction which consists overall of seeking a source block in a source sub-band in order to predict a target block of the sub-band which it is sought to code. For this purpose, it is necessary to construct a dictionary of possible source blocks, generally containing all the blocks which exist in a window of the source sub-band and source blocks formed by applying a transformation (multiplication by a constant, geometric transformations, for example) to the preceding source blocks.
The target block is next compared with all the source blocks in the dictionary, and then the closest source block is selected as a coded form of the target block.
This method makes it possible to obtain good visual quality of the restored image and limited distortion. However, the time required for processing each target block, as disclosed above, and consequently the time required for coding an image, is lengthy.
The present invention aims to remedy the drawbacks of the prior art by providing a device and method for compressing a digital signal which offers rapid coding, with a quality of restoration of the image which is at least equal to that obtained with known methods.
To this end, the invention proposes a digital signal coding method including a step of analysing the digital signal into a plurality of frequency sub-bands distributed in at least two different resolutions, at least one first sub-band having a lower resolution and at least one second sub-band having a higher resolution,
characterised in that it includes, for each second sub-band the steps of:
dividing the second sub-band into target blocks,
selecting, for each of the target blocks, a predetermined number of source blocks in a first sub-band,
determining, for each of the target blocks, a transformation between the source blocks selected at the preceding step and the target block.
Correlatively, the invention proposes a digital signal coding device including means of analysing the digital signal into a plurality of frequency sub-bands distributed in at least two different resolutions, at least one first sub-band having a lower resolution and at least one second sub-band having a higher resolution,
characterised in that it includes, for each second sub-band:
means of dividing the second sub-band into target blocks,
means of selecting, for each of the target blocks, a predetermined number of source blocks in a first sub-band,
means of determining, for each of the target blocks, a transformation between the source blocks selected at the preceding step and the target block.
The method and device according to the invention make it possible to code a digital signal rapidly, whilst offering a high ratio of compression to distortion.
Advantageously, the invention proposes a digital signal coding method including a step of analysing the digital signal into a plurality of frequency sub-bands distributed according to at least two different resolutions, at least one first sub-band having a lower resolution and at least one second sub-band having a higher resolution,
characterised in that it includes, for each second sub-band, the steps of:
dividing the second sub-band into target blocks,
selecting target blocks which are to be coded by setting to a predetermined value, according to a selection criterion,
coding the target blocks selected at the preceding step, by setting to the predetermined value,
selecting, for each of the target blocks which have not been selected, a predetermined number of source blocks in a first sub-band,
determining, for each of the target blocks which have not been selected, a transformation between the source blocks selected at the preceding step and the target block.
Correlatively, the invention proposes a digital signal coding method including a step of analysing the digital signal into a plurality of frequency sub-bands distributed according to at least two different resolutions, at least one first sub-band having a lower resolution and at least one second sub-band having a higher resolution,
characterised in that it includes, for each second sub-band:
means of dividing the second sub-band into target blocks,
means of selecting target blocks which are to be coded by setting to a predetermined value, according to a selection criterion,
means of coding the target blocks selected at the preceding step, by setting to the predetermined value,
means of selecting, for each of the target blocks which have not been selected, a predetermined number of source blocks in a first sub-band,
means of determining, for each of the target blocks which have not been selected, a transformation between the source blocks selected at the preceding step and the target block.
The coding of selected blocks by setting to a predetermined value makes it possible to improve the ratio of compression to distortion.
According to a preferred characteristic, the selection step includes, for each of the target blocks, the coding of the target block by setting to the predetermined value and by determining a transformation between source blocks and the target block, the comparison of the two coding modes according to the selection criterion and the selection of coding by setting to the predetermined value if the block coded by setting to the predetermined value satisfies the selection criterion.
Selection is thus effected simply and rapidly.
According to another preferred characteristic, the selection criterion minimises a weighted sum of the transmission rate and coding error caused by the coding of the target block under consideration. The criterion is reliable and simple to implement.
According to a preferred characteristic, an indicator is associated with each target block in order to indicate whether or not the block under consideration is coded by setting to the predetermined value.
According to a preferred characteristic, for any target block under consideration, the location of the source blocks selected in the first sub-band is determined according to the location of the target block in the second sub-band. The invention therefore entails no seeking of a source block, which saves time during coding.
According to a preferred characteristic, for any target block under consideration, the source blocks selected in the first sub-band overlap in pairs. This makes it possible to improve the quality of the coding.
According to preferred characteristics:
the sub-bands formed at the analysis step contain details in different orientations with respect to the original digital signal,
for any target block under consideration, the relative position of
the source blocks selected in the first sub-band depends on the orientation of the sub-band under consideration, and
for any second sub-band under consideration, the first sub-band is the sub-band of the same orientation and with the resolution immediately below with respect to the second sub-band.
These characteristics improve the quality of the coding by supplying more xe2x80x9cpertinentxe2x80x9d, source blocks for coding the target block.
According to other preferred characteristics, for any target block under consideration, the transformation is a multilinear approximation and minimises a distance between the target block and its approximation calculated by applying the transformation to the selected source blocks. The calculation of the distance includes the calculation of a difference between the values of the data of the target block and the values of its approximation calculated by applying the transformation to the source blocks.
The coding thus entails only simple and rapid calculations.
According to a preferred characteristic, the distance is the root mean square error calculated between the values of the target block and the values of the block resulting from the application of the approximation to the source blocks.
According to another preferred characteristic, for any one of the target blocks under consideration, the source blocks have a size which is a multiple by a factor F2 of that of the target block and are sub-sampled by the factor F, where F is an integer greater than or equal to 2, or the source blocks have the same size as the target block.
The invention also concerns a method of decoding a coded digital signal, the said signal including coded representations of blocks formed in at least one second sub-band of a plurality of frequency sub-bands of the original signal, the said sub-bands being distributed according to at least two different resolutions, at least one first sub-band having a lower resolution and the said at least one second sub-band having a higher resolution, each coded representation including at least one transformation between the block and selected source blocks in the said at least one first sub-band,
characterised in that it includes, for each block of each second sub-band, the step of applying the transformation to the source blocks in order to decode the block.
The invention also concerns a method of decoding a coded digital signal, the said signal including coded representations of blocks formed in at least one second sub-band of a plurality of frequency sub-bands of the original signal, the said sub-bands being distributed according to at least two different resolutions, at least one first sub-band having a lower resolution and the said at least one second sub-band having a higher resolution, each coded representation including at least one indicator for indicating whether the block has been coded by setting to a predetermined value or by a transformation between the block and selected source blocks in the said at least one first sub-band,
characterised in that it includes, for each block of each second sub-band, the steps of:
reading the indicator,
forming a block, all the coefficients of which are at the predetermined value if the block was coded by setting to the predetermined value,
applying the transformation to the source blocks in order to decode the block, if the block was coded by a transformation between the block and source blocks.
Correlatively, the invention proposes a device for decoding a coded digital signal, including means of implementing these characteristics.
The decoding method and device make it possible to reconstruct the signal, for example in a receiving apparatus corresponding to a sending apparatus in which the signal has been coded according to the invention.